Extreme strontium concentrations reveal specific biomineralization pathways in certain coccolithophores with implications for the Sr/Ca paleoproductivity proxy.

The formation of the coccolith biominerals by a group of marine algae (the Coccolithophores) offers fascinating research avenues both from the biological and geological sides. It is surprising how biomineralisation by a key phytoplanktonic group remains underconstrained, yet is influential on ocean alkalinity and responsible for the built up of our paleoclimatic archive over the last 200 Myrs. Here, we report two close relative coccolith taxa exhibiting substantial bioaccumulation of strontium: Scyphosphaera and Pontosphaera grown in the laboratory or retrieved from Pliocene sediments. This strontium enrichment relative to calcium is one order of magnitude greater than reported in other coccoliths of the orders Isochrysidales and Coccolithales, and extends well beyond established controls on Sr/Ca ratios by temperature and growth rate. We discuss this prominent vital effect in relation with possible specific uptake of strontium relative to calcium from the extracellular environment to the coccolith vesicle in coccolithophores excreting very large scale coccoliths. The report of Sr-rich biominerals challenges our current understanding of the cellular acquisition and intracellular trafficking of alkaline earth cations in phytoplanktonic calcifying eukaryotic algae. The presence of Sr-rich coccolith species in the geological record has to be quantitatively considered in future Sr/Ca-based palaeoceanographic reconstruction.

Fossil evidence for serpentinization fluids fueling chemosynthetic assemblages.

Among the deep-sea hydrothermal vent sites discovered in the past 30 years, Lost City on the Mid-Atlantic Ridge (MAR) is remarkable both for its alkaline fluids derived from mantle rock serpentinization and the spectacular seafloor carbonate chimneys precipitated from these fluids. Despite high concentrations of reduced chemicals in the fluids, this unique example of a serpentinite-hosted hydrothermal system currently lacks chemosynthetic assemblages dominated by large animals typical of high-temperature vent sites. Here we report abundant specimens of chemosymbiotic mussels, associated with gastropods and chemosymbiotic clams, in approximately 100 kyr old Lost City-like carbonates from the MAR close to the Rainbow site (36 °N). Our finding shows that serpentinization-related fluids, unaffected by high-temperature hydrothermal circulation, can occur on-axis and are able to sustain high-biomass communities. The widespread occurrence of seafloor ultramafic rocks linked to likely long-range dispersion of vent species therefore offers considerably more ecospace for chemosynthetic fauna in the oceans than previously supposed.